The invention pertains generally to the field of FM stereophonic multiplex transmission and has particular reference to a FM radio receiver capable of receiving more than a single mode of transmission, including, e.g., monophonic, biphonic (two channel stereophonic) and quadraphonic (four channel stereophonic) broadcasts. More specifically, the invention pertains to the IF filter circuit portion of such FM radio receiver.
FM stereophonic broadcast has been defined by the Federal Communications Commission as the transmission of a stereophonic program by a single FM broadcast station utilizing the main channel and a stereophonic subchannel. In accordance with the present FCC FM broadcast standard, the main channel is composed of the band of audio frequencies from 50 Hz to 15 KHz, and the stereophonic subchannel is provided in the band of frequencies from 23 to 53 KHz, containing a suppressed subcarrier at 38 KHz and its associated 15 KHz upper and lower sidebands. The main and subchannel signals frequency modulate the main carrier along with a 19 KHz pilot signal which is employed to reconstitute the suppressed subcarrier in the receiver. By the same token and for purposes of the present discussion, FM quadraphonic broadcast may be defined as the transmission of a quadraphonic program by a single FM broadcast station utilizing the main channel, the stereophonic subchannel and two additional subchannels. One such system is described in U.S. Pat. No. 3,934,092, entitled "Four Channel Stereophonic System", for the invention of Antal Csicsatka, which is assigned to the assignee of the present invention. In this system a third subchannel is provided in the band of frequencies from 23 to 53 KHz, which is a suppressed subcarrier, double sideband modulation in quadrature with the existing stereophonic subchannel. A fourth subchannel is provided in the band of frequencies from 61 KHz to slightly above 76 KHz, containing a suppressed subcarrier at 76 KHz and a vestigial sideband modulation. A 76 KHz pilot signal is also transmitted for indicating at the receiver the presence of a quadraphonic transmission and for providing an automatic operation of the receiver's quadraphonic decoder.
Monophonic FM receivers, capable of receiving only monophonic broadcasts, normally provided an IF bandwidth of about 150 KHz, which figure is measured at the 3 db attenuation points. A bandwidth of this order establishes a compromise in providing a low distortion signal at the receiver output through the inclusion of a sufficient number of the higher order sidebands in the rf spectrum, without admitting excessive noise or interfering signals from adjacent channels. For two channel stereophonic receivers which must accomodate a wider modulation band of stereophonic transmission, it is necessary to increase the IF bandwidth. A bandwidth of about 180 KHz at the 3 db attenuation points is employed in most present day FM stereophonic receivers. Since stereophonic receivers are also capable of receiving monophonic broadcasts, opening the IF bandwidth to this extent does produce some degradation of the received monophonic signal by interfering signals, which for all but high performance equipment is normally within tolerable limits.
Quadraphonic receivers, however, in order to reproduce a low distortion signal from the further increased modulation band of quadraphonic transmission, must provide a substantially wider IF bandwidth, in the order of 230 KHz at the 3 db attenuation points. A bandwidth significantly less than this would create distortion and reduce separation for the quadraphonic signal at the receiver output. A wider bandwidth, on the other hand, would not be of benefit because the transmitted signal is restricted to an IF bandwidth of such magnitude, in order to satisfy Federal Communications Commission requirements directed to avoiding adjacent channel interference.
By increasing the receiver IF bandwidth to 230 KHz, two principal problems are introduced in the reception of monophonic and two channel stereophonic broadcasts, which the quadraphonic receiver must be able to receive as a compatibility requirement. The first of these is adequacy of protection ratio. For purposes of the present discussion, protection ration, which is related to receiver selectivity, is a figure of merit that compares the magnitude of the desired signal voltage from a selected channel with the magnitude of undesired signal voltages from any other channels that are present at the receiver input. It may be expressed as the ratio in db of an undesired carrier signal voltage to the desired carrier signal voltage, where the undesired carrier signal produces an audio signal of 30 db attenuation with respect to the audio signal produced by the desired carrier signal. The increased IF bandwidth of the quadraphonic receiver is found to cause a notable reduction in protection ratio. Thus, with the receiver switched to the monophonic receiving mode, the widened IF bandwidth may subject the receiver to interference from undesired signals, which may be monophonic, biphonic or quadraphonic signals transmitted on other than the selected channel. Similarly, the widened IF bandwidth may also result in interference from such undesired signals when the quadraphonic receiver is switched to receive in the two channel stereophonic mode. On the other hand, the receiver need not be subject to interference from undesired signals when in the quadraphonic receiving mode because the reception range of quadraphonic transmission is substantially less than that of either monophonic or two channel stereophonic transmission, which acts to compensate for the reduced selectivity of the IF filter bandwidth.
As to the second problem posed by the increased IF bandwidth, the total noise introduced into the IF stage of the quadraphonic receiver will be increased during reception in the monophonic and two channel stereophonic modes due to the widened IF bandwidth, resulting in a degraded overall receiver performance when operating in either of these modes.
No practical solution to the above noted problems is known to exist for quadraphonic receivers. Although techniques for providing an automatic adjustment of filter bandwidth are, in general, known to the art, no circuitry is known to have been evolved which could be used for the present application or which would have the advantages of the present invention.